1. Field of the Invention
This invention relates to a magneto-optical recording medium such as magneto-optical disk and card, a manufacturing method for the medium and a magneto-optical recording and playback device to record and play back data using the magneto-optical recording medium. This invention specially relates to a magneto-optical recording medium having a recording layer comprising a garnet ferrite layer and being ideal for high density and proximity recording, a manufacturing method for the medium and a magneto-optical recording and playback device for the medium.
2. Background Art
In conventional magneto-optical recording media on the market, the recording layer is mainly made from a thin metal layer. Data are recorded onto the recording layer as recorded bits by changing the optical properties such as the transmission or reflection rate of minute spots on the recording layer by a light beam for recording data. An amorphous alloy of rare earth metals such as TbFeCo is a typical material for the thin metal layer, and the alloy is favorable for recording data because it generally has a high absorption coefficient (>105 cm−1). However, the thin metal layer is prone to deterioration due to oxidation and other factors. For this reason, it has to be sealed and protected by a plastic layer, for example.
On the other hand, a magneto-optical recording medium having a recording layer that consists of an oxide such as garnet ferrite, which is a kind of ferrite having a garnet type structure of crystals and has a large Faraday rotation angle, has been developed. In such a recording medium, the degree of deterioration of properties due to oxidation is smaller than that of the case in which a metal material is used for the recording layer, because the material itself of the recording layer is already an oxide. Therefore, it has the distinctive feature that the above mentioned special protection is not necessary.
In the case in which garnet ferrite is used as the material for magneto-optical recording, however, internal stress occurs in a garnet ferrite layer when spattering for formation of the layer on a substrate. This sometimes results in cracks in the garnet ferrite layer, rough morphology of the surface of the layer and very large crystal particles, which are not preferable because they cause the problem of medium noise when recording and playing back. In order to overcome the above-mentioned problems, a method for improvement of the morphology of the layer by adjusting the thermal expansion coefficient of the substrate and by performing inverse spattering, for example, after annealing, is disclosed in Japanese Patent Publication No. Hei 8-249740 (1996).
Meanwhile, a new type of recording layer having a plurality of layers made from several kinds of materials for magneto-optical recording has recently been developed for the purpose of improvement of the S/N and C/N ratio. However, it is said that metal materials such as the above amorphous alloy are not suitable for multi-layered structures since they have a relatively small Kerr Effect and a high absorption coefficient. As a result, a recording layer having multiple-layers made from garnet ferrite, which is known as an oxide-type material for optical recording and which has large Faraday Effect, has attracted much attention. For instance, a multi-layered recording medium having piled layers made from bismuth-substituted type garnet ferrite having an excellent Faraday rotation angle in the wavelength range of visible light has been proposed (see Itoh, Koike, Numata, Inoue and Kawanishi “Multi-Layered Magnetic Garnet Ferrite Film for Magneto-Optical Recording”, Abstract of 10th Academic Lectures for Application of Magnetics in Japan, p. 31, November 1986).
However, a multi-layered recording medium having a recording layer made from bismuth-substituted garnet ferrite requires a high power light beam when writing data because the absorption coefficient of the recording layer is small. Taking this problem into consideration, Japanese Patent Publication No. Hei 6-282868 discloses a multi-layered type magneto-optical recording medium having a light absorption layer which accelerates the recording process by efficient transfer of the applied by a light beam to a recording layer which is near the light absorption layer. However, the above mentioned magneto-optical recording medium uses direct energy gap semiconductors such as GaAs, InP, CdS, CdSe, ZnSe and ZnS, which are easily oxidized during layer formation, as the light absorption layer. Therefore, a protective layer is required on the surface of the light absorption layer. The protective layer is formed by deposition of SiO2, Al2O3, TiO2 or the like in the range of 2˜100 μm by a CVD method or spattering to create a film. Accordingly, the manufacturing process shown in Japanese Patent Publication No. 6-282868 becomes complex because formation of the light absorption layer and the protective layer is necessary in addition to formation of the recording layer, which leads to increased production costs. Besides, properties such as the S/N ratio of the magneto-optical recording medium manufactured by the above method have not yet reached a satisfactory level.
Japanese Patent Publication No. Hei 6-290497 (1994) discloses a manufacturing method for a magneto-optical recording medium having a recording layer that has a double-layered garnet ferrite structure, in which a non-magnetic garnet ferrite underlayer is used, and discloses that the multi-recording layer keeps the garnet ferrite crystal particle diameter at 1 μm or below to reduce the disparities in bit shapes and medium noise. However, the above method is impractical because the manufacturing process is complex. Further, in the case in which the multi-layered structure is formed by two types of garnet ferrite having different compositions, the elements in each garnet ferrite layer disperse at the vicinity of layer boundaries after heat treatment. Therefore, compositional deviations occur in the direction perpendicular to the layer surface in the multi-layered structure, which causes the problem of deterioration of properties such as the S/N ratio and repeatability. Furthermore, high density recording is hindered if the above mentioned method is employed because it is impossible for the above mentioned garnet ferrite layer formation method to provide minute crystals on the order of nanometers.
On the other hand, various approaches to the improvement of the S/N and C/N ratios have been examined from not only the aspect of the magneto-optical recording medium itself but also from the aspect of devices for recording and playing back data on the magneto-optical recording medium.
The method to realize high resolving power by raising the optical refraction rate by fulfilling a liquid between a sample and an object lens is known. An application of this method using a small solid lens has been proposed under the name of SIL lens (Appl. Phys. Lett., 57(24), 1990; U.S. Pat. No. 5,004,307). And, a data recording and playback system using the SIL lens for magneto-optical recording medium has been also proposed (U.S. Pat. No. 5,125,750). This system is characterized in that the distance between the SIL lens and the recording layer of the recording medium is kept within the wavelength of the light, namely on the sub-micron order, so as to gain a small focus spot diameter which is a feature of the SIL lens, and so as to gain an increase in the recording density. However, even in this system, the S/N and C/N ratios have not yet reached a satisfactory level. Besides, this system lacks generality because it is not suitable for a magneto-optical recording medium that has a relatively thick protective layer, which is, for example, disclosed in Japanese Patent Publication No. Hei 6-282868.